In semiconductor manufacturing, operations wafers and their carriers are handled by robots and manufacturing process equipment. This creates static charges on the wafers and carriers. These charges can cause wafer damage through arcing to ground and contamination when airborne particles are attracted to the wafer. Wafers are held in cassettes which need to be changed when moving to or from a process environment. To change cassettes, wafers are lifted out of one cassette by comb like fingers made of insulative material which project into a cassette from below, and are then lowered into a new cassette which replaces the old cassette.
In the process of lifting wafers out of a first cassette, the wafers become highly electrically charged when extricated from the cassette because of decreasing capacitance relative to ground paths. If this charge is not removed, highly charged wafers will be placed in the new cassette and enter a new process environment in a condition where damage to the wafer could occur by sparking or arcing.
Many times manufacturers of wafer and wafer carrier handling equipment use static dissipative plastic components to make contact with the wafers. While this does reduce static charges, the materials currently available shed particles causing wafer contamination at the contact point. Manufacturers are seeking alternatives to this approach.
FIGS. 1-3 illustrate the above method by which wafers become charged. A cassette 11 has internal ribs 13 which loosely support a wafer 15. Cassettes have standard dimensions designed to hold a plurality of wafers, sometimes 25 or so, for simultaneous processing through a variety of machines, such as thin film coating devices, ovens, inspection and measuring devices, and other chip manufacturing machines. The cassette 11 is open at the top and bottom but inwardly turned skirt regions 17 near the bottom provide the lowermost support for the wafers, preventing wafers from dropping through the cassette. Full details of a wafer transfer machine may be seen in U.S. Pat. No. 5,193,969 to Rush et al., assigned to Fortrend Engineering Corporation.
The cassette 11 is placed in a wafer transfer machine 21 so that the wafers can be lifted and supported from above while a second cassette replaces the first one. The machine 21 has a support plate 23 on which cassette 11 may rest, guided by side rails 25. A pair of wafer rack members 27 project through the support plate 23. The rack members 27 are spaced apart, insulative, polymer members which have a comb shape with upwardly projecting teeth, allowing wafers to be positioned between teeth. Motor controlled metal shafts 28 support the rack members 27 with a wafer lifting operation initiated by an operator pushing a button 30. The objective is to raise the wafer to a gantry 29 where arms 31 will hold the wafer 15 while the lifters are retracted to a position below support plate 23 and another cassette is placed in replacement of cassette 11.
As a wafer is raised to the position shown in FIG. 3, capacitance of the wafer relative to ground changes substantially compared to the capacitance in the wafer configuration of FIG. 1. Charge induced onto the wafer in its lower position is now isolated on the wafer. The voltage associated with the isolated charge increases as capacitance of the wafer relative to ground decreases. Voltages of several kilovolts are common. With a new cassette in place, the lifters are raised until the rack members 27 contact the wafer, arms 31 are released, and the wafers are lowered into the new cassette. However, the high voltages now on the wafers have the potential of damaging wafers upon entry into a process environment.
An object of the invention was to provide an apparatus for electrostatic discharge of wafers in wafer transfer operations.